Enabling Remote Connections to SQL Server Express
By default, SQL Server Express will only allow connecting from a local
client. To enable remote connections, there are two settings to
change:
1) First, select the properties of the SQL Server Express instance
within Microsoft SQL Server Management Studio and make sure that "allow
remote connections to this server is checked".

2) Now open the SQL Server Surface Area Configuration tool and change
the remote connections to allow both local and remote connections.